In the past, significant efforts have been made to develop high resolution, high speed printing systems. Such efforts have resulted in high resolution devices but at high cost. Accordingly, efforts have been made to devise systems which can achieve a high resolution image, yet are low in cost to manufacture.
One technology which has been promising is high voltage electrostatic printing. In an electrostatic printing device, a write head having a plurality of stylii is positioned in spaced-apart relationship to a dielectric coated paper substrate. A voltage is applied to the write head sufficient to exceed a breakdown voltage of the air in the stylus to paper gap in order to ionize the air and deposit a charge on the dielectric surface of the paper. The paper is then passed by a toner station having oppositely charged ink particles, where it will pick up an image-wise pattern of toner in correspondence with the charges applied to the surface. The toner can then be fused to the surface of the recording medium by any one of several methods known in the art.
One method of obtaining high resolution, high speed printing via an electrostatic printing process is to use an array of conductive wires arranged in linear fashion across the width of the paper. With this approach, there is no need for complex mechanical structures to move the write head relative to the paper. Rather, it is simply necessary to move the paper past the write head in a direction orthogonal to the printhead.
One of the shortcomings with the latter approach is that it is quite difficult to provide a low cost system for addressing a high number of densely packed wires. In a typical high resolution printer, the wires are formed in an array of 400 wires per inch across the typical 36-inch width of the printhead such that over 14,000 wires will be addressed. Commercially available printers utilizing high-voltage electrostatic technology require several hundred expensive high voltage transistor drive circuits, together with a complex multiplexing scheme to accomplish reasonable resolution/speed performance.
Various other schemes have been proposed in the prior art for addressing the ends of the conductive writing elements or wires in an electrostatic printing process. In the mid-1950's, A. B. Dick Company began developing an electrostatic printing process where a cathode ray tube was used to sequentially address the ends of a wire matrix for transferring charge to a dielectric recording medium. In this device, called the Videograph, an electron beam was scanned over one end of the wires with the electrons being transferred along the wires through the front surface of the cathode ray tube and into a stylus array writing head. The charge was then deposited on dielectric paper by the process discussed previously. The Videograph proved to be a very successful device for printing mailing labels and the like, however, the cathode ray tube could not provide a low cost mechanism for large arrays.
The subject invention utilizes another method for supplying current to the wires coming from the write head. In the subject system, each wire is provided with a layer of a photoconductive material, whose resistance changes when addressed with a beam of radiation. This change in resistance can be used in an electrical circuit to provide a voltage at the write head.
Photoconductive materials have been used in electrostatic printing devices for some time. A high voltage electrostatic printing technology known in the art as simultaneous charge transfer has been used with photocopiers. A description of this type of device can be found in "Charge Transfer Electrophotography", Jepsen and Day, Photographic Science and Engineering, 1974. In this device, a glass substrate is coated with a transparent conductive layer. The transparent conductor is overlaid with a photoconductive layer. The substrate is held in a spaced-apart relationship to a sheet of dielectric coated paper. A voltage is applied to the conductive layer that would be sufficient to ionize the air gap between the photoconductive layer and the surface of the paper.
In operation, light is passed through a negative image to be copied, such as microfilm, and impinges on the photoconductive surface through the transparent conductive coating. In local areas where light strikes the photoconductor, resistivity of the photoconductive layer will drop, permitting the current in the conductive layer to raise the voltage at the photoconductor surface above the breakdown voltage such that an electric charge is deposited in image-wise fashion on the paper. The paper is then passed to a toner station where the image is developed.
As can be appreciated, the latter device was designed simply to make copies, that is, to take analog information on one media and transfer it to another media in analog form. Efforts were made to adapt this type of technology for converting digital data into printing. One such attempt was described in "Laser Recording on Dielectric-Coated Paper Using Simultaneous Charge Transfer", Day et al., Proceedings First European Electro Optics Markets and Technology Conference, Geneva, Switzerland, IPL Technology Press Ltd., p. 363, September 1972. In the device described in the latter article, a laser beam was directed inside a rotating glass drum having an outer photoconductive structure identical to the device discussed above. The laser was modulated with the digital data and scanned across the inner surface of the drum. While appearing feasible, these efforts proved too costly and no commercial product was ever released.
In the literature there are many examples of printers using photoconductors. For example, U.S. Pat. No. 2,898,468, issued Aug. 4, 1959 to McNaney, discloses a device which has a printhead adjacent the printing surface. A pair of electrodes are provided, one of which is connected to high voltage and the other to a writing stylus. These electrodes are separated by a photoconductor having high resistance in the dark. When this photoconductor is struck by an energy beam, its resistance is reduced, permitting current to pass between the two conductors in a manner to deposit a charge on the moving substrate.
There have been continuing efforts to develop electrostatic printers which utilize a combination of a printhead, defined by a plurality of wires, and a photoconductive excitation technique. Such a device is described in U.S Pat. No. 3,689,933 to Klose. In this low voltage device, elements formed from a photoconductive material are arranged in a circular target zone. These elements are addressed using a laser beam and a laser beam modulator. More specifically, the laser beam is passed through an optical shutter and then to a beam deflector. This beam is scanned in conical fashion to address a photoconductor which is deposited in a circular format on a flat substrate. The plurality of wires extend radially from the scanned circle and are placed in direct contact with a dielectric drum onto which charge is deposited through a direct ohmic contact. The wires are energized with a low voltage circuit in a manner similar to the device disclosed in McNaney patent. This Klose device was also never commercialized.
The subject invention utilizes a similar method for supplying current to the wires coming from the write head. In the subject system, each wire is provided with a layer of a photoconductive material, whose resistance changes when addressed by a beam of radiation. This change in resistance is then used in an electrical circuit to provide a switched voltage on the stylus.
As can be seen in previous examples of prior art, laser scanners have been identified as useful devices for high density printing systems. A problem associated with designing a high density scanner is that misalignment of the optics can result in timing errors. More specifically, in order to transfer the charges to the correct location on the paper, the radiation beam must be focused on the proper photoconductive element at the proper time or the image will be displaced or otherwise distorted. Accordingly, it would be desirable to provide a new and improved optical scanner which can be used for high density printing and can be easily adjusted to remove timing errors.
Many attempts have been made to develop optical scanners which would satisfy the above requirements. These scanners usually employ the property of reflection to redirect a radiation beam to produce a scanned beam. The redirection is achieved by changing the angle between the plane of the reflective element and the incident radiation beam in a prescribed way. For example, the scanner can be arranged such that the axis of the incident energy beam and the rotational axis of the reflecting member are perpendicular. Devices which perform this function include motors mounted with a mirrored polygons, galvanometer drive assemblies with a mounted single mirror and resonant torsional assemblies also with a mounted single mirror.
These configurations require that the reflecting surface become longer in one dimension as the scan angle increases. The effect of this lengthening is generally to keep this class of scanners to scan angles of 90.degree. or less. The corresponding angular excursion of the scanning mirror would be 45.degree. which would result in an increase in the effective size of the mirror to 1.4 times that of the beam diameter. Where data is to be decoded from a motor shaft to give information about the angle at which the reflected radiation beam is being directed, the accuracy required is based on the complete rotation divided by the scans per revolution and further divided by the number of pixels or writing elements per scan. In these devices, a full scan is created with less than a full rotation of the scanning mirror, such that there is a high degree of pointing accuracy required. This problem is made worse by the double angle which is generated upon reflection.
One approach which has been taken to overcome the latter problems includes the development of circular scanners. In these devices, a mirror is attached to a motor shaft at 45.degree. to the rotational axis of the motor. A radiation beam impinging upon the mirror will produce a scanning pattern that is generally conical, but usually nearly planar. If conical, these devices can scan a circle on a plane and if conical or planar they can scan a circumferential target zone on the inside of a cylinder or other geometric surface of revolutions. Examples of circular scanners can be found in U.S. Pat. Nos. 3,875,587, issued Apr. 1, 1975 to Pugsley, and 3,651,256, issued May 21, 1972 to Sherman et al. The subject invention provides an improved low cost circular scanner which is applicable to high resolution printing.
As discussed above, in many electrostatic printing systems, a photoconductive material is irradiated with a beam of energy such that the resistance of the material is reduced. The material is connected to a voltage source, such that the drop in resistance changes the current flow into an associated stylus, producing a voltage change in the stylus. This voltage will rise until it exceeds the breakdown voltage of the air layer between the writing stylus and the dielectric recording media. Once the voltage has increased above this breakdown level, charge will be deposited on the surface of the dielectric. The amount of charge placed on the paper will depend in part upon the length of time the light source is focused on the photoconductor. Once the laser is turned off, the resistance of the photoconductor will begin to rise, slowing the current flow into the wire, and eventually reducing the voltage at the gap below the breakdown level. Prior to that time, however, the circuit will still be conducting and depositing charge on the substrate.
In a similar fashion, some printers rely on direct ohmic contact of the wires to the dielectric recording media. In the latter case, charge is deposited at a much lower voltage as there is no layer of air that has to be ionized before charging will begin. In both cases, where dielectric coated paper is moving past the printhead, if the decay time of the photoconductor is too slow, the area of the deposited charges will elongate. When the image is toned, the contrast ratio will be reduced and the ink will appear smeared. Therefore, it is desirable to place some kind of control on the decay time of the photoconductive material.
An attempt at addressing this problem is disclosed in U.S. Pat. No. 3,466,657, issued Sept. 9, 1969, to Rice. Rice discloses a matrix printer including 5 by 7 wires each one of which was connected to a photoconductive driver. These wires would be energized in a simultaneous manner to form a character. A plurality of masks were provided, each mask having a particular printing pattern. A light source would be energized to illuminate individual masks to produce the desired characters.
Each mask in the Rice device was provided with a companion mask that exposed the complements of the wires of the first mask. The second mask includes a circuit where the wires were shunted to ground. Thus, each time the radiant energy is supplied, the wires for printing are energized and the wires which would not print are being discharged. This had the effect of ensuring that no residual charges built up on the wires. By reducing the residual charges, it was felt that the decay time could be minimized. Unfortunately, the Rice patent did not disclose a means for adjusting the rate of decay. Indeed, the device in Rice did not even affect the wires which were activated during the write cycle. Therefore, it would be desirable to provide a device where the conductance of a second photoconductor, connected to the same writing stylii as the first photoconductor, could be controlled to effect the decoy time of the actuated wires as well as to control the voltage of the wires which have not previously printed.
Accordingly, it is an object of the subject invention to provide a new and improved electrostatic printing device.
It is another object of the subject invention to provide a new and improved optical scanner for an electrostatic printer.
It is a further object of the subject invention to provide an optical scanner which can be readily adjusted to remove timing errors.
It is another object of the subject invention to provide a new and improved electrostatic printer which can produce high resolution and print speed at low cost.
It is still a further object of the subject invention to provide a new and improved electrostatic printer where the write time of the stylii can be controlled while keeping the duty cycle of the scanner high.